Method for preventing myopia progression through identification and correction of optical aberrations

ABSTRACT

A method for at least one of preventing myopia and retarding the progression of myopia is provided. The method includes measuring optical aberrations in a human eye (42, 43, 44) and correcting the optical aberrations (46). Measuring optical aberrations (42, 43, 44) may include measuring wavefront aberrations (45) of parallel light rays entering the eye.

TECHNICAL FIELD

The present invention pertains to measuring optical aberrations in thehuman eye and, more particularly, to preventing myopia and retarding theprogression of myopia by correcting the aberrations.

BACKGROUND ART

Myopia (or near-sightedness) has become the most pervasive visualdisorder in the world. About twenty-five percent of people inindustrialized countries in the Western world, and more than fiftypercent of people in industrialized Asian countries require opticalcorrection for myopia. With increasing educational demands, theprevalence of myopia is increasing steadily. Extensive reading bychildren and adolescents appears to cause progressive myopia. Sinceincreasing educational demands have increased the prevalence of myopia,optical correction, such as eyeglasses, contact lenses, and refractivesurgery for myopia is a major health care expense.

Myopia is due primarily to an elongation of the posterior pole of theeye during the school age years. Structures in this region of the eyetend to be stretched during development, and their integrity iscompromised. This causes greater risk to the effects of ocular trauma,diabetes, macular degeneration, and other diseases. This means thatmyopia is also a major contributor to irreversible blindness.

Referring to FIG. 1, in a normal eye, the cornea 10 and lens 12 at thefront of the eye focus an image of the visual world on the retinalreceptors 14 at the back of the eye. At the retinal receptors the imagebegins to be processed and sent on to the brain as a complex neuralsignal. A myopic eye is too long, so that the image of most of thevisual world is focused in front of the retina. Consequently, myopia istreated by weakening the optical power in the front of the eye so thatthe image is focused on the retina. This means that eyeglasses, contactlenses and refractive surgery are not treating the basic disorder, butare merely counteracting the effects of ocular elongation. Each of thesetreatments has its own problems, is expensive, and in no way reduces thelikelihood that the myopic person will contract one of the blindingdiseases which are secondary to myopia later in life.

Eyeglasses, contact lenses, and to a lesser extent, refractive surgerycan accurately correct myopic defocus (often referred to as thespherical error of the eye) by placing as much of the focused image aspossible on the retina. Some eyes have an aberration that creates adifference in optical power between one meridional orientation andanother. This aberration is known as astigmatism and is correctable witheyeglasses (although eyeglasses cause visual distortion) and withspecialty contact lenses (which may be uncomfortable).

At least thirty other “higher order” aberrations can be measured andquantified in the human eye. Each of these aberrations contributes adifferent type of degradation to the retinal image. These aberrationsare usually measured in the laboratory with a complex optical instrumentin which a laser beam is aimed at the retina of people who have theirpupils dilated with drugs. However, such aberrations can now be measuredin children and adolescents without the use of bright light or the needfor pupillary dilation with drugs.

Traditional clinical correction of optical defocus places the averageposition of an image of the visual world on the retina. However, partsof that image may be in front of or behind the retina due to therefractive properties of the eye's aberrations. Thus, most of the imagein a “perfectly” corrected eye may be significantly out of focus due tothese aberrations.

A small number of people have myopia due to rare inherited diseases or,in old age, in conjunction with diabetic crystalline lens changes. Morethan ninety percent of the people with myopia, however, develop itduring their school age years. It has been shown that this progressivemyopia is clearly related to a genetic predisposition (Pacella et al.,“Role of Genetic Factors in the Etiology of Juvenile-Onset Myopia Basedon a Longitudinal Study of Refractive Error,” Optom. Vis. Sci. 76,381-386, (1999)) and to an intensity of school work, especially reading.

Animal studies have shown conclusively that blurring the visual world byscattering an image through the use of eyelid closure or smoked orsandblasted eyeglass lenses leads to myopia. Similarly, defocusing thevisual world with minus lenses induces myopic response. Both blurring(i.e., general image degradation) and myopigenic defocus affectsmyopigenesis more in some species than in others and more in some breedsthan in others within the same species. This suggests that the myopicresponse to environmental influences is genetically dependent.

Epidemiological studies and other studies demonstrate the same type ofenvironmental-genetic interaction

SUMMARY OF THE INVENTION

A method is provided for preventing myopia and/or retarding theprogression of myopia by measuring optical aberrations in a human eyeand correcting the optical aberrations.

In accordance with one embodiment of the invention, the step ofmeasuring includes measuring wavefront aberrations of parallel lightrays entering the eye. The step of measuring may also include measuringdeviations at the retina of parallel light rays entering the eye, aswell as measuring deviations at the pupil of parallel light raysentering the eye.

In accordance with another embodiment of the invention, the step ofmeasuring may include providing a multi-channel optical system whereinan aperture is moved to multiple positions with respect to the pupil ofthe eye and alignment parameters for each aperture position is recordedat least one optical distance. A system of equations is then solved toderive a set of aberration constants based on the alignment parameters.The step of measuring may also include detecting first and higher orderastigmatism and/or detecting at least one of first and higher ordercoma, spherical aberrations and other aberrations.

In accordance with a further embodiment of the invention a method forpreventing myopia and/or retarding the progression of myopia includesscreening for aberrations in a human eye, measuring the aberrations andcorrecting the aberrations. The step of screening may include detectinga depth of focus by measuring visual acuity, contrast sensitivity and/orblur sensitivity and the visual acuity, contrast sensitivity and/or blursensitivity may be measured with a psycho-physical test.

In accordance with other embodiments of the invention, the stepcorrecting may include providing an optical device, providing at leastone optical lens and/or providing at least one contact lens. The step ofcorrecting may also include altering an optical surface in the eyeand/or performing corneal surgery. The step of correcting may furtherinclude providing intra-ocular implants.

In accordance with additional embodiments of the invention, the step ofcorrecting includes providing adaptive optics, and the adaptive opticsmay include deformable mirrors, systems of multiple lenslettes,micro-mirror electro-machined components, optically addressed liquidcrystal spatial light modulators, membrane mirrors, and/or piezoelectricbi-morph mirrors. The adaptive optics may produce periods of clearvision and may be miniaturized so as to be wearable on the face of aperson

The step of correcting may also include providing high illuminationlevels to reduce the pupil of the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of an eye relevant to the present invention;

FIG. 2 is a flow chart representing the steps involved in a method ofthe present invention;

FIG. 3 is a flow chart representing another embodiment of the presentinvention;

FIG. 4 is a flow chart representing an embodiment of the invention thatemploys an apparatus such as a multi-channel optical device;

FIG. 5 is flow chart representing an embodiment of the invention whereinaberrations of a human eye are screened by determining depth of focus;

FIG. 6 is a schematic of a multi-channel optical device that may be usedto perform the measurements of the present invention;

FIG. 7A shows a front view of a prior art pupil sampling aperture; and

FIG. 7B shows a pattern of entry positions at the pupil of the subjectof a sampling beam using the apparatus of FIG. 7A.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The inventors have shown that myopic children and adolescents tend tounder accommodate when looking at near targets during and prior to theperiod in which they are developing myopia. This under accommodationresults in a myopigenic defocus similar to that which induces myopia inanimal experiments. The inventors have also demonstrated that childrenand adolescents who are becoming myopic have certain binocular anomalies(near esophoria and high AC/A ratio) which tend to cause underaccommodation (Gwiazda, J., Thorn, F., Bauer, J. and Held, R., “MyopiaChildren Show Insufficient Accommodation to Blur,” Invest. Ophthalmol.Vis. Sci., 34, 690-694 (1993); Gwiazda, J., Bauer, J., Thorn, F., andHeld, R., “A Dynamic Relationship Between Myopia and Blur-DrivenAccommodation in School Aged Children,” Vision Res., 35, 1299-1304(1995); Gwiazda, J., Grice, K., and Thorn, F., “Response AC/A Ratios areElevated in Myopic Children,” Physiol. Optics., 19, 173-179 (1999) allof which are incorporated herein be reference). This suggests thatmyopigenic defocus due to under-accommodation induces myopia in childrenand adolescents.

When children must look intensely at near patterns, for example, duringreading, their eyes accommodate (focus) and converge (turn in together)on the text being read. This accommodation effort tends to increaseoptical aberrations, causing increased blur, and the eyes tend to underaccommodate, causing a myopigenic defocus. These innate factors, plusextensive reading in children and adolescents whose eyes are still youngenough to grow, appear to cause progressive myopia.

The inventors have shown that all adults with high amounts of opticalaberrations are myopic. In fact, about 25% of myopic children and adultshave optical aberrations that are greater that those in non-myopicadults. The aberrations in myopic eyes were often two or three times aslarge as the upper limit in non-myopic adults.

In accordance with the present invention, it is taught that if largeaberrations in the eyes of children are detected and treated, theprogression of myopia may be retarded or eliminated.

Definitions and proposed standards relating to measurement of opticalaberrations are available in (Atchison et al., “Mathematical Treatmentof Optical Aberrations: A User's Guide,” Trends in Optics and Photonic,Optical Society of America, 35, 110-130 (2000) and Thibos et al.,“Standards for Reporting the Optical Aberrations of the Eyes,” Trends inOptics and Photonics, Optical Society of America, 35, 232-244 (2000)which are hereby incorporated herein by reference.

FIG. 2 illustrates a method for preventing myopia and/or retarding theprogression of myopia. Optical aberrations are measured in step 21.Aberrations may be expressed in terms of Zernike polynomials; however,the use of other representations is within the scope of the invention.The optical aberrations are then corrected in step 22 using opticalcorrecting devices well known in the art including, but not limited to,spectacles, contact lenses, adaptive optics, corneal surgery, lasersurgery, and intra-ocular implants.

FIG. 3 illustrates a method for preventing myopia and/or retarding theprogression of myopia wherein deviations, at a pupillary or retinallocation, of parallel light rays (e.g., 16 in FIG. 1) entering a humaneye are measured in step 31. The optical aberrations are thencalculated, step 32, from the measurements taken in the previous stepand the aberrations are corrected in step 33 with optical correctiondevices as described in connection with FIG. 2.

FIG. 4 illustrates a procedure for measuring wavefront aberrations usinga three channel system as shown in FIG. 6. The procedure is discussed inHe et al., “High Optical Quality is a Necessary Condition for the HumanEye to Maintain Emmetropia,” (1999), which is attached hereto andincorporated herein by reference. Referring to FIG. 6, the system hasseparate channels 61, 62, 63 for test, fixation-stimulus, andpupil-monitoring respectively. A pupil of a subject is located at P₀ anda retina of the subject is located at R₀. A Badal optometer (focusingblock) 64 allows an operator to change the refractive state of the testchannel 61 and the pupil-monitoring channel 62 together, withoutchanging the location of the pupil conjugate planes (P₁, P₁′, P₂ andP₂′) and retinal conjugate planes (R₁, R₂, R₂′, and R₃)

A 543-nm He—Ne laser 60 produces light for the test channel 61. Thecoherence of the laser 60 is broken by a rotating diffuser 65. The lightfrom the laser 60 is collimated by a lens 66 and 12 mm steel ball 68.The reflection from the ball 68 produces a divergent, high-numericalaperture beam 67 that the subsequent optics image as a point source. Agimbaled mirror 69 is controlled by an analog joystick (not shown) thatallows the subject to change the angle of the mirror 69 rapidly in twodimensions. Tilting the mirror 69 changes the angle at which the testbeam enters the eye and therefore changes the retinal location of thetest spot.

At the pupil-monitoring channel 62, a pupil entry position of the testbeam is selected from a set of 1 mm holes 72 (shown in FIG. 7A) thattile the pupil of the eye by rotating a aperture metal wheel 70 (seeFIG. 7A) that is optically conjugate to the pupil. The aperture wheel 70is constructed such that it can be rotated to one of 37 presetlocations.

A fixation target, typically a cross, is provided at a fixation-stimuluschannel 63. The fixation-stimulus channel 63 is illuminated by a lightsource, such as a fiber-optic illuminator 75. Light from the illuminator75 is collimated and then passes through a filter holder-slide holder 74located in a retinal conjugate plane R₂′. The light from the illuminator75 is then imaged on an adjustable iris diaphragm 76 located in a pupilconjugate plane P₁. The iris diaphragm 76 is set to 1 mm diameter tomatch the size of the pupil sampling. However, for conditions in whichthe wavefront properties of the eye are measured when the eye isaccommodated by high illumination levels, the diameter is increased to 6mm to provide a better stimulus. The fixation-stimulus channel 63 iscombined with the pupil-monitoring channel 62 at beam splitter 77.

In step 41 of FIG. 4, the subject's eye is first aligned to the opticalaxis of the system by using an infrared sensitive CCD video camera 78.The camera 78 provides a magnified view of the pupil. By looking at amonitor screen of a computer (not shown) and adjusting the Badal system64 to clarify the screen, the eye is at its resting state. Measurementsare referenced to the entry location within the pupil. The measurementsconsist of a few practice trials and six tests, three for each eye. Instep 42 an aperture 72 is moved to multiple positions with respect tothe pupil to produce a pattern as shown in FIG. 7B. A test may consistof thirty-nine trials with the first and the last trials for the centerof the pupil. The other thirty-seven trials randomly sample the entirepupil with a 7×7 matrix in 1 mm steps except the twelve points in thefour corners. The subject's task is to align a cursor with the center ofthe fixation target and click a mouse of the computer on each trial.Each test usually lasts about three minutes, and the entire sessionrequires approximately thirty minutes.

In step 43, the shifts in the fixation target are recorded by thecomputer and translated into the slope of the wavefront at thethirty-seven pupil locations. In step 44 a system of equations is solvedusing a least square procedure to fit the slope measurements to thederivative of thirty-five terms of the Zernike polynomial functions. Thederived coefficients provide estimates of the weights of the individualaberrations, and are used, in step 45, to reconstruct the overallwavefront at the pupil plane. The aberrations are corrected in step 46in the same manner as described in step 22 of FIG. 2.

Procedures and devices for measuring wavefront aberrations are furtherdiscussed in Liang et al., “Objective Measurement of Wave Aberrations ofthe Human Eye With the Use of a Hartmann-Shack Wave-Front Sensor,”Journal of the Optical Society of America A, 11, 1-9 (1994); Thibos,“Principles of Hartmann-Shack Aberrometry,” Trends in Optics andPhotonics, Optical Society of America, 35, 163-169 (2000); and He et al,“Measurement of the Wave-Front Aberration of the Eye by a FastPsychophysical Procedure,” Journal of the Optical Society of America A,15, 2449-2456 (1998) each of which is incorporated herein by reference.

FIG. 5 illustrates a method for retarding the progression of myopiawherein a human eye is screened for aberrations. Visual acuity, contrastsensitivity, and/or blur sensitivity are measured in step 51. Depth offocus is determined in step 52 from the measurements performed in step51 using procedures described in Thorn et al., “Myopia Adults SeeThrough Defocits Better than Emmetropes,” Myopia Updates, Springer,Tokyo, 368-374, T. Tokoro (ed.) (1998) and Rosenfield and Abraham-Cohen,“Blur Sensitivity in Myopes,” Optometry and Vision Science, 76, 303-307(1999) which are also incorporated herein by reference. Deviations inparallel light rays (e.g., 16 in FIG. 1) entering a human eye at theretina or pupil of the eye are measured in step 53. Aberrations are thencalculated, step 54, from the measurements taken in the previous step.The aberrations are precisely measured as described above and thencorrected in step 55.

Optical correction for aberrations within the human eye may be providedthrough several different optical procedures. In one embodiment of theinvention, spectacle lenses are used to reduce astigmatic aberrations.In another embodiment of the invention, contact lenses are used toreduce second and third, and perhaps higher orders of aberrationsbecause contact lenses move with the eye, thereby preserving thealignment of their optical surfaces with the optical surfaces of the eyeduring eye movements. See, (Bartsch et al., “Resolution Improvement inConfocal Scanning Laser Tomography of the Human Fundus,” TechnicalDigest Series 2, Optical Society of America, 2, 134-137 (1994) andGuirao et al, “Effect of Rotation and Translation on the ExpectedBenefit of Ideal Contact Lenses,” Trends in Optics and Photonics,Optical Society of America, 35, 324-329 (2000) which are incorporatedherein by reference.

Lenses may also be used to change (usually reduce) optical accommodationlevels in order to reduce optical aberrations. Several studies have usedthis procedure for various purposes. See, for example, tine study byJackson and Brown, “Progression of Myopia in Hong Kong Chinese SchoolChildren is Slowed by Wearing Progressive Lenses,” Optometry and VisionScience, 76, 346-354 (1999) which is incorporated herein by reference.

It is also within the scope of the invention to perform corneal surgeryto reduce optical aberrations. This procedure is described in Hamam, “AQuick Method for Analyzing Hartmann-Shack Patterns: Application toCorneal Surgery,” Trends in Optics and Photonics, Optical Society ofAmerica, 35, 187-198 (2000); Hong and Thibos, “Optical AberrationsFollowing Laser in Situ Keratomileusis (LASIK) Surgery,” Trends inOptics and Photonics, Optical Society of America, 35, 220-226 (2000);and Munger, “New Paradigm for the Treatment of Myopia RefractiveSurgery,” Trends in Optics and Photonics, Optical Society of America,35, 227-230 (2000) all of which are incorporated herein by reference.

Further, intra-ocular implants a may be used to reduce opticalaberrations. The rationale, measurement, and analysis used for theintra-ocular implant embodiment is the same as that in the refractivesurgery reduction of aberrations discussed above.

In another embodiment adaptive optics are used to reduce opticalaberrations. Adaptive optics may employ deformable mirrors, micro-mirrorelectro-machined components, lenslette arrays, optically addressedliquid crystal spatial light modulators, membrane mirrors, orpiezoelectric bi-morph mirrors to correct the eye's aberrations.Adaptive optics may be used in devices that can be worn when they areminiaturized to the point of wearability. Adaptive optics devices mayalso be used in instruments that allow patients to experience periods ofclear vision through reduced optical aberrations. See, for example,Roorda and Williams, “Adaptive Optics and Retinal Imaging,” Trends inOptics and Photonics, Optical Society of America, 35, 151-162 (2000) andMunger, “New Paradigm for the Treatment of Myopia Refractive Surgery,”Trends in Optics and Photonics, Optical Society of America, 35, 227-230(2000) each of which is incorporated herein by reference.

In another embodiment high illumination levels are used to reduce pupilsize thereby reducing the amount of optical aberrations. This method isdiscussed in Campbell, “Contributions to the Optical Quality of the Eye:Implications for ‘Perfect’ Optical Correction,” Trends in Optics andPhotonics, Optical Society of America, 35, 227-230 (2000) which is alsoincorporated herein by reference.

It should also be noted that the embodiments described herein are notmutually exclusive and can be used in combination. For example, visualacuity, contrast sensitivity and/or blur sensitivity may be measured incombination with measurement of wavefront aberrations. Likewise, thenumerous devices mentioned above in connection with correcting theaberrations may be used in combination with one another to produceequivalent or superior results.

Although the above embodiments are preferred, many modifications andrefinements which do not depart from the true spirit and scope of theinvention may be conceived by those skilled in the art. It is intendedthat all such modifications, including but not limited to those setforth above, be covered by the following claims.

High Optical Quality is a Necessary Condition for the Human Eye toMaintain Emmetropia

-   Ji C. He*, Pei Sun^(§), Richard Held*, Frank Thorn*, Editha Ong*,    Xiuru Sun^(§), Jane E. Gwiazda* *New England College of Optometry,    424 Beacon Street, Boston, Mass. 02115, USA^(§) Institute of    Psychology, Chinese Academy of Science, P.O. Box 1603, Beijing,    Beijing 100012, P. R. China

Vision is optimized when the focal plane of the eye's optics iscoincident with the retina so that the image of a distant object fallson the photoreceptor layer: a condition called emmetropia. A mismatchbetween the focal and axial lengths of the eye causes refractive errorsin the forms of either hyperopia (far-sightedness), when the focal planelies behind the retina, or myopia (near-sightedness), when it is infront of the retina. Most children's eyes approach emmetropia at about 5years of age from a mismatch in infancy¹⁻². While many children maintaintheir emmetropia into adulthood, others become myopic because the eyerows too long. Animal studies indicate that degrading image quality cancause myopia³⁻⁸. A similar causation for the human eye is less clear.Human eyes recently have been found to have irregular aberrations⁹⁻¹⁶,which degrade image quality, thereby making them candidates formyopization. We measured monochromatic aberrations in myopic andemmetropic children and adults, and found that adult emmetropes had lessaberrations than either myopes or emmetropic children. These resultsindicate that high image quality is necessary for maintainingemmetropia.

A single lens forms an image of a distant object at its focal plane. Thedistance between the lens and the focal plane is the focal length, acharacteristic parameter of the lens. The focal length of the human eyeis determined by both the geometric curvature of corneal and lenssurfaces and the refractive indices of their ocular media. In mostinfants, the focal length is greater than the axial length so that thefocal plane lies behind the retinal plane (hyperopia). Eye growth inchildhood tends to match the focal plane with the retina so as toachieve emmetropia. The match, however, is not maintained in animalexperiments if image clarity is disrupted by either lid fusion³⁻⁵ orotherwise depriving the eye of spatial information⁶⁻⁸. Thesemanipulations cause the eye to grow too long so that the focal planelies in front of the retina (myopia). This dependence of myopiadevelopment on image quality has been observed in various speciesranging from chicken to monkey, but the underlying mechanisms are notfully understood. Although experimental manipulation on the human eye isnot possible, Nature poses its own tests of this issue. The human eye isnot an ideal optical system. Its defects are called aberrations and arecaused by local variations in both surface curvatures and refractiveindices in the cornea and the lens and/or misalignment of the opticalaxes of the cornea and lens relative to the visual axis of the eye. Theaberrations cause the light rays passing through the pupil to divertfrom their ideal paths, and proportionately degrade image quality sothat clearest vision can not be reached even though the focal planematches the retina perfectly. Recent measurements in human eyes haveshown that the aberrations vary substantially from one individual toanother in their form and amount⁹⁻¹⁶. In this study we measuredaberrations for 280 subjects with different ages and differentrefractive errors in order to demonstrate the effect of aberrations,hence image quality, on the match between the focal plane and the retinain the human eye.

Wavefront aberrations at the pupil plane have recently been used tocharacterize the overall effect of aberrations^(9-11, 13-16). Thewavefront represents an equal-phase surface for the light rays passingthe pupil at any given time, and forms a flat surface on the pupil planeif the eye is ideal. Deficiencies in the optics of the eye cause thewavefront to deviate from the ideal surface, and the degree of thewavefront deviations, or wavefront aberrations, directly depends on howthe optics are flawed. We used a psychophysical ray-tracing techniquewith natural pupils to measure wavefront aberrations^(9, 16), and used aroot-mean-square (RMS) of the deviated wavefront, relative to the idealflat wavefront as an estimate for the effect of wavefront aberrations.Subjects were divided into four groups according to their age andrefractive error as shown in Table 1. Among the 280 subjects, eighteenpercent are Caucasian and eighty-two percent are Chinese.

Frequency histograms of the RMS of wavefront aberrations in the worsteye for each subject indicate that every subject has RMS ofwave-aberration greater than 0.5. This result means that the human eyeis not perfect but suffers image degradation resulting from thedeficiency in optics. Adult emmetropes, however, have the lowest meanRMS of wave-aberration which is significantly different from the meansin the other groups (vs children's emmetropic group, t=5.55 p<0.0001; vsmyopic adult group, t=4.85, p<0.0001; and vs myopic children's group,t=6.45, p<0.0001). They have the smallest standard deviation and it isalso significantly different from the other groups (vs emmetropicchildren's group, F=2.39, p<0.005; vs myopic adult group, F=8.89,p<0.001; and vs myopic children's group, F=10.72, p<0.001). The highestRMS value in the adult emmetropic group is 1.62 which is exceeded byforty percent of myopic adults, 37.5% of myopic children, and 23% ofemmetropic children in our sample. The results indicate that adultemmetropes suffer the least image degradation and that stronger imagedegradation occurs for about forty percent of myopes who haveaberrations greater than all adult emmetropes.

Compared with the wave-aberrations for emmetropic children, some of whommay develop myopia later, the limited wave-aberration for adultemmetropes indicates that for the human eye to maintain emmetropia imagedegradation must be small. People with strong wave-aberration, whosuffer strong image degradation, may fail to maintain the match betweenthe focal plane and the retinal plane, and thereby develop myopia. Thesefindings are in agreement with the evidence from animal studies.

It has been suggested that myopia causes aberration¹⁵, but if theelongation of the eye in myopia generally caused optical deficiencies inthe cornea and lens, we would expect all or most myopes to have moreaberrations than emmetropes. But this is not true for the sixty percentof myopes who have aberrations no greater than the adult emmetropes.

Genetic contributions to myopia have long been recognized¹⁷⁻¹⁹, but theunderlying mechanisms are unclear. Aberrations of the eye caused bydefects in the cornea and lens may be inherited. Thus aberrationscausing image degradation may be one of the genetic mechanisms leadingto myopia. Meanwhile, the role of near-work can not be excluded.Stronger aberrations have been reported for an accommodated eye²⁰⁻²¹.Near-work would expose the eye to stronger image degradation and thusimpose a higher risk of developing myopia. Besides the contribution ofaberrations either inherited or near-work associated, the existence ofsixty percent of myopes with less aberrations necessarily indicates acontribution from other factors on myopia development.

Small aberrations with reduced variability in adult emmetropes, as foundin this study, suggest that high optical quality of the image is anecessary condition for the human eye to maintain emmetropia. Ourresults also suggest that severe aberrations are associated with thedevelopment of myopia. The existence of strong aberrations in myopia,which can not be corrected with available techniques, necessitates thedevelopment of new techniques for vision care in clinical practice. Theresults also provide important information about the optics of the humaneye for designing experiments in vision research and visual instrumentsin the optical industry.

Methods

Apparatus. The apparatus used in this study is a three channel opticalsystem, including a test, a reference and a pupil monitoring channel,which share the design of the subjective wavefront sensor described in aprevious study¹⁶ in principle but was changed to a computer monitorversion. The test channel provides a green cross target on the retinavia a movable aperture with 1 mm diameter, As the aperture is moved fromtrial to trial among 37 locations within the subject's natural pupil,the cross shifts its retinal location accordingly due to the aberrationof the eye. The cross shifts were traced by the subject via a cursor onthe monitor of a computer provided in the reference channel. Thesubject's pupil was monitored by a CCD camera and a monitor in thepupil-monitoring channel during the experiment and any eye movementrelative to the optical axis of the system was compensated by moving a3D translator on which the subject's head rested. In the systems thereis a movable stage with two mirrors on the common pathway set as a Badelsystem for compensating the subject's refractive error.Procedure. The subject's eye was first aligned to the optical system. Bylooking at the monitor screen via a 1 mm aperture and adjusting theBadal system to clarify the screen, the eye was at its resting state.The measurements consisted of a few practice trials and six tests, threefor each eye. Each test consisted of 39 trials with the first and thelast trials for the center of the pupil. The other 37 trials randomlysampled the entire pupil with a 7×7 matrix in 1 mm steps except the 12points in the four corners. The subject's task was to align the cursorwith the center of the cross and click the mouse on each trial. Eachtest usually lasted about 3 minutes, and the entire session took about ahalf hour. Data Analysis. The shifts in the cross target recorded by thecomputer were translated into the slope of the wavefront at the 37 pupillocations. A least square procedure was used to fit the slopemeasurements to the derivatives of 35 terms of the Zernike polynomialfunctions. The derived coefficients provide estimates of the weight ofindividual aberrations, and were used to reconstruct the overallwavefront at the pupil plane.

REFERENCE

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TABLE 1 Subjects' information Number of Subjects Age (years) Sphericalequivalent Male Female Mean Range error (Diopter) Emmetropic 20 25 21.519-27 0.75 to −0.5 Adults Emmetropic 45 35 15.1 12-17 0.75 to −0.5Children Myopic Adults 39 41 21.4 19-29 −0.6 to −9.0 Myopic Children 3540 15.0 11-18 −0.6 to −7.0Figure Caption

FIG. 1 Frequency histograms of the root-mean-square (RMS) of wavefrontaberration in the human eye for 280 subjects in four groups. The numberof subjects (N) and the mean RMS with standard deviation are indicatedfor each group.

1. A method for at least one of preventing myopia and retarding theprogression of myopia, the method comprising: identifying a child proneto myopia on the basis of aberrations in an eye of the child; measuringoptical aberrations in the child's eye; and impeding progression ofmyopia by correcting the optical aberrations.
 2. A method according toclaim 1, wherein the step of measuring includes measuring wavefrontaberrations of parallel light rays entering the eye.
 3. A methodaccording to claim 1, wherein the step of measuring includes measuringdeviations at the retina of parallel light rays entering the eye.
 4. Amethod according to claim 1, wherein the step of measuring includesmeasuring deviations at the pupil of parallel light rays entering theeye.
 5. A method according to claim 1, wherein the step of measuringincludes providing a multi-channel optical system.
 6. A method accordingto claim 5, further comprising: moving an aperture to multiple positionswith respect to the pupil of the eye; recording alignment parameters offeatures for each aperture position at least one optical distance; andsolving a system of equations to derive a set of aberration constantsbased on the alignment parameters.
 7. A method according to either ofclaim 5 or 6, wherein the step of measuring includes detecting first andhigher order astigmatism.
 8. A method according to either of claim 5 or6, wherein the step of measuring includes detecting at least one offirst and higher order coma, spherical aberrations and otheraberrations.
 9. A method for at least one of preventing myopia andretarding the progression of myopia, the method comprising: screeningfor higher order aberrations in an eye of a child; measuring theaberrations in the eye; and impeding progression of myopia by correctingthe aberrations.
 10. A method according to claim 9, wherein the step ofscreening includes measuring visual acuity.
 11. A method according claim9, wherein the step of screening includes measuring blur sensitivity.12. A method according to claim 10, wherein visual acuity is measuredwith a psycho-physical test.
 13. A method according claim 11, whereinblur sensitivity is measured with a psycho-physical test.
 14. A methodaccording to either of claim 1 or 9, wherein the step of correctingincludes providing an optical device.
 15. A method according to eitherof claim 1 or 9, wherein the step of correcting includes providing atleast one optical lens.
 16. A method according to either of claim 1 or9, wherein the step of correcting includes providing at least onecontact lens.
 17. A method according to either of claim 1 or 9, whereinthe step of correcting includes altering an optical surface in the eye.18. A method according to either of claim 1 or 9, wherein the step ofcorrecting includes performing corneal surgery.
 19. A method accordingto either of claim 1 or 9, wherein the step of correcting includesproviding intra-ocular implants.
 20. A method according to either ofclaim 1 or 9, wherein the step of correcting includes providing highillumination levels to reduce the eye's pupil.
 21. A method according toeither of claim 1 or 9, wherein the step of correcting includesproviding adaptive optics.
 22. A method according to claim 21, whereinthe adaptive optics include deformable mirrors.
 23. A method accordingto claim 21, wherein the adaptive optics include at least one system ofmultiple lenslettes.
 24. A method according to claim 21, wherein theadaptive optics include micro-mirror electro-machined components.
 25. Amethod according to claim 21, wherein the adaptive optics includeoptically addressed liquid crystal spatial light modulators.
 26. Amethod according to claim 21, wherein the adaptive optics includemembrane mirrors.
 27. A method according to claim 21, wherein theadaptive optics include piezoelectric bi-morph mirrors.
 28. A methodaccording to claim 21, wherein the adaptive optics are miniaturized soas to be wearable on the face of a person.
 29. A method according toclaim 21, wherein the adaptive optics produce periods of clear vision.30. A method according to claim 9, wherein the step of screeningincludes measuring contrast sensitivity.
 31. A method according to claim30, wherein contrast sensitivity is measured with a psycho-physicaltest.
 32. A method according to claim 9, wherein the step of screeningincludes measuring depth of focus.
 33. A method according to either ofclaim 1 or 9, wherein the step of measuring includes measuring wavefrontaberrations using an aberrometer.
 34. A method according to either ofclaim 1 or 9, wherein the step of measuring includes measuring wavefrontaberrations using a Shack-Hartman wavefront sensor.